In polarization multiplexing transmission, to reduce signal degradation caused by polarization dependent loss (PDL) of the optical fiber, the processing of rotating the polarization of optical signals has been conventionally executed to average the effect of polarization dependency of the optical fiber. As a result of the reduction of signal degradation caused by PDL, it is unnecessary for allowing for excessive margins for various design values in transmission systems adopting the polarization multiplexing transmission, enabling saving of device costs. Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2011-146795 and 2010-109705.
Japanese Laid-open Patent Publication No. 2010-109705 describes that, to demodulate a received signal, the receiver-side digital signal processing unit gives a change of the polarized wave whose characteristics are substantially inverse to the change of the polarized wave in the transmitter side signal processor to the digital electrical signal, but the means include various complicated calculations. This method disadvantageously increases loads of demodulation processing. Japanese Laid-open Patent Publication No. 2011-146795 fails to specifically describe demodulation processing.
In consideration of the above-mentioned situation, there is a demand for providing a transmission device, a reception device, and a transmission method that can reduce signal degradation caused by PDL without increasing loads of demodulation processing.